Solid state starters/controllers have found widespread use for controlling application of power to an AC induction motor. The conventional starter/controller, referred to hereinafter as simply a starter or a controller, uses solid state switches for controlling application of AC line voltage to the motor. The switches may be thyristors such as silicon controlled rectifiers (SCRs) or triacs.
One application for a motor controller is as an elevator starter. The elevator starter may be used to drive a pump for an hydraulic elevator. Each time movement of an elevator car is commanded, then the starter must start the motor until it reaches operating speed and then operate in a run mode. Such a starter may only be used for the up direction as gravity may be used for the down direction.
One type of elevator starter initially connects the motor windings in a Y-configuration to start the motor and bring it up to speed. Then the windings are reconnected in a delta configuration with full voltage. Other starters, referred to as soft starters, change the on time of the solid state switches to control voltage and to ramp up motor current with a fixed connection. Known elevator starters have selector switches for setting a starting current limit setting. Depending on configuration, the setting is adjustable from about 100 percent to 450 percent of the starter's current rating. The time required to bring a motor up to speed with a current limit start is a function of the difference in the torque provided at the current limit setting and the torque required to accelerate the load or pump. As a general rule, the higher the current limit setting the lower the start time and conversely, the lower the current limit setting the longer the start time. In an elevator application end users are interested in starting the motor as fast as possible.
In applications where the load during the start is light and doesn't vary from start to start, for example a hydraulic elevator motor, current limit is generally the preferred method to start the motor. However, if the torque required to start the load or pump increases, then the start time will increase. Depending on the torque required to start the load or pump, it may take an unsuitably long time to get the motor up to speed. To compensate for this time delay, known soft starters increase the motor start current past the current limit setting if the motor does not come up to speed in an allotted time. Some starters use a fixed time or a variable time based on the average start time. The delay time enables the rotor to accelerate up to the appropriate speed before additional current boost is given under normal conditions. This routine works well when the torque is sufficient to allow the rotor to continue to accelerate during the allotted time frame. If the load is not up to speed in the allotted time, then the boost provided when the starter increases the current typically brings the rotor up to speed. However, there can be exceptions that result in failure of the motor start operation.
As long as the torque provided by the current limit setting remains higher than the torque required to accelerate the load or pump throughout the torque curve, the motor will continue to accelerate until it increases to an appropriate speed. If at any time during the start operation the torque provided by the current limit setting is equal to the torque required to accelerate the load, then the motor will no longer be able to accelerate and will remain at constant speed until the current boost algorithm is implemented. During this time the motor will continue to spin at the “stalled” speed. In applications where the starting current is around 200% of the motor's full load current, the average start time may exceed 2 to 2.5 seconds. This can result in a delay of up to 5 seconds before the starter begins to boost the current in an effort to bring the motor up to speed. The time spent in the delay is wasted as the motor is spinning at the same low speed when the time delay expires as it was when the stalled condition was encountered, regardless of the length of the delay.
The present invention is directed to solving one or more of the problems discussed above, in a novel and simple manner.